HYDRIDE INTERFERENCE ON THE DETERMINATION OF MINOR ACTINIDE ISOTOPES BY INDUCTIVELY-COUPLED PLASMA-MASS SPECTROMETRY

Hydrogen adducts of the major naturally occurring actinide isotopes Th-232 and U-238 were studied using an inductively coupled plasma mass spectrometer. The hydride:atomic ion ratios for both elements varied as a function of the parameters that were studied, i.e., nebulizer flow rate, solution uptake rate and desolvation conditions. When the instrument sensitivity for U and Th was optimized, (ThH+)-Th-232:Th-232(+) was found to be (3.9 +/- 0.2) x 10(-5) with pneumatic nebulization and (2.10 +/- 0.07) x 10(-5) with ultrasonic nebulization. Under the same conditions, (UH+)-U-238:U-238(+) was found to be (3.2 +/- 0.2) x 10(-5) and (1.8 +/- 0.1) x 10(-5) using pneumatic and ultrasonic nebulization, respectively. Conditions that reduced hydrogen number density and/or increased plasma temperature decreased the hydride:atomic ion ratio. Such conditions are best if U-233 and Pu-239 are to be determined in the presence of Th-232 and U-238.